Merrill McPeek is largely responsible for this.
The AF use to have an overwhelming number of Officers in Joint Billets, derisively called the blue wave, McPeek cut those slots as part of the Clinton draw down insisting that the AF not provide more than 25% (IIRC) of the manning. The Army (and Navy to some extent) filled the void.
The AF also seems to have developed a career track where certain types of assignments are for the A-Team, and everyone else gets pushed into the remaining slots.
I agree that a ground force commander may be better. This could also be a way to shake up "group think" by bringing a semi-outsider in (or he needs the assignment so he can be a future AFSOC Commander, which ties into my Sugar Daddy comment). We also don't know what they see as emerging/future threats.
Has MARSOC matured to the point of providing a Senior Leader with a lot of SOF experience? LTC/COL billets maybe, but I would question a SOF GO appointment (you could argue that STO/CRO could fill a GO Billet at JSOC based on their ground/air experience).